Activation of cellulose



Patented Mar. 10, 1953 2,631,144 ACTIVATION F cELLULosE Lejaren Arthur Hiller, Jr.', Waynesboro, Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 23, 1950, I

Serial No. 191,737

The present invention relates to the, activation of cellulose by heat preparatory to basic catalyzed esterification. More particularly, the invention relates to the oven drying of cellulose at an elevated temperature whereby the cellulose is activated for subsequent acetylation with acetic anhydride in the presence of potassium acetate and optionally in the presence of an organic compound such as dimethylformamide.

By basic catalyzed is meant the catalysis of the esterification using such substance as sodium, potassium, and lithium acetate.

It is commonly recognized in the cellulose ester art that for optimum results cellulose needs to be activated prior to its esterification with fatty acid anhydrides and catalysts. This treatment is referred to ordinarily as pre-treatment, pre-soaking or activation. In addition to chemicalpretreatments, either by acidic or alkaline processes, one of the efiective ways which has been disclosed in the prior art of increasing cellulose reactivity for acetylation has been to use a dwelling treatment. An example of such technique is the ,water activation method which is frequently used in preparing cellulose for acid catalyzed es.- terification. In this particular process, the cellulose structure is distended by first allowing it to swell in water. The water held in the cellulosic structure is then replaced with a reagent such as acetic acid through several successive washings therewith. As a result, a. highly reactive substance is swollen with a reagent which is normally employed as a diluent in acetylations catalyzed by strong acids.

The attractive possibilities of prior art teach ings, such as the one mentioned above, would appear offhand to lend themselves to use in th'ebasic catalyzed procedure; Yet such is not thecase. Even though this technique has been modified so as to be better adapted for the basic catalyzed process, i. e., dimethylformamide has been substiutted for acetic acid as a reagent to replace water after swelling, the swollen cellulose instead of being more completely acetylated reacts to a lesser extent. In order for the rapid basic catalyzed procedure to compete commercially with the acid catalyzed process, a simple, inexpensive and efiicient pre-treatment is desirable.

7 Claims. (01. zen-229i It is, therefore, an object of this invention to I wetting the cellulose.

tained in about 45 minutes. ,not given the drying treatment; but was treated 2 butyrolactone, and gamma-calerolactone. Another object of the invention is to providea simple yet expedient method of activating cellulose which does not involve excessively long periods of time, nor the typing up of expensive equipment. A further object is to provide a process of drying cellulose preparatory to esterification in which the moisture content of the cellulose may be reduced below 2% or even 1% of moisture without deleteriously affecting the cellulose in any way, while simultaneously enhancing its reactivity. A still further object of the invention is to provide for a process of activating cellulose seemingly independent of water removal and free of exacting conditions of time and temperature. Other objects of the invention appear herein.

In its broadest aspects, the invention comprises heat treating cellulose, preferably in an oven, at an elevated temperature for an appreciable period of time prior to esterification with an acid anhydride'in the presence of a basic catalyst and optionally in the presence of certain organic compounds claimed in co-pending cases Serial Nos. 224,829 and 224,830 whereby the reactivity of cellulose is enhanced for subsequent esterifica tion. It has been further found that it is unnecessary to keep the cellulose anhydrous between the pre-treatment and the actual esterification, for the increased reactivity is not lost upon re- Still further, swelling is not essential in the process of this invention.

A typical process in accordance with the invention is one in which a quantity of cellulose pulp was divided into two parts, one part was placed in an oven at C. and thoroughly dried therein. After about 20 hours of drying, the cellulose is acetylated by refluxing it in a solution containing 150.00. of acetic anhydride and cc. of N,N-dimethylformamide (DMF), and 5 g. of potassium acetate. Instead of the expected 3 or 4 hour reaction time, a reaction product having a combined acetic acid content of 58% is ob- The second part was after a total reaction period of four hours the combined acetic acid content of the ester does not approach 58% but remains in the neighborhood of 53%. If the basic catalyst is omitted the esterifications are very slow and incomplete.

{The following examples, given for illustrative purposes only, show in greater detail the pretreatment of cellulose in accordance with the invention:

Example I A quantity of wood pulp was steeped in water, then divided into approximately two equal batches. One-half of the material was allowed other half was not rewetted. Samples of anhydrous and rewetted cellulose (5 parts by weight) were subjected to a series of basic catalyzed acetylations under the conditions shown in Table to dry at room temperature; the other half was 5 III whmh follows oven-dried at 120 C. for a period of about 20 TABLE III hours. In a series of tests five parts by weight of room temperature, air-dried material (control) and 5 parts by weight of the oven-dried Conditions), Acetic Potasi c% d i batch were subjected to the acetylating condim Cellulose $5, 2 DMF g tions described in Table 1, shown below. All re- 30 Min. 1 Hr. actions were carried out for a period of one hour at the reflux temperature. The quantities shown finhygitnfils g g in the following tables are parts by weight unless We a 5 otherwise indicated. ttltll 2i Anhydrous 22 123 2. 5 56. l 58. 9 TABLE I Rewetted 22 123 2.5 55.3 59.0

Acetic 0811111058 e- Percent The series of acetylations described in the above Anhy- DMF ffj g fF 0mm? table (Table III) demonstrate clearly the nondride ype of Acetic Drying) Acid essent1alness of maintaining the cellulose in an anhydrous condition during the period from oven 162 0 2.5 Room Temp. 36.3 drying till the commencement of esterification. 8 3:3 ,232 Temp ggg While the invention has been described above 162 0 10.0 Oven 55.7 25 with reference to the activation of cotton linters g} i} 3325 Temp 22:? and wood pulp cellulose, it is to be realized that g; R 1 Temp. thiihis clones fortconvegience stake olnly, inasmugh as e prerea ment ereina er 0 aimed may e 33 5321? Temp' 2812 applied to all types of cellulose esterifiable by the basic catalyzed process. Due to the eifectiveness The data Shown in the above table clearly of the oven treatment, a better product will be onstrate the activation resulting from oven dry- Obtaine? upon acetylation regardless of the type ing of wood .pulp preparatory to basic catalyzed feSte1'1fia b1e cellulose used acetylation. Activation at the oven temperature Tlfle actlYated cellulose obtamed by pljactlce 0 is faster and leads to greater results. Further, 35 the mventlon may be employed for making other the heat treatment is effective in the absence of eters than cellulose acfata'tei cellulose the organic compounds used in copending cases Plonatei cemllose proplonate'butymte and the seria1NoS 224829 and 22433Q like. In making the latter type of derivative, at

least a large proportion or even the entire an- E-w p 40 hydride may be propionic or butyric anhydride A second series of reactions were performed as theesierifying agent As these anhydl'ides are using cotton linters. As in the previous example, more sluggis in t e acetylating ct t n acetic one-half of the material was not heat-treated prior anhydride, is important that e 0611111059 to acetylation under the conditions shown below ployed therem be highly reactivein Table While the other half was dried in It is to be understood that all basic catalysts an oven at Q for Sixteen hours prior to 15 with or without the organic activators which being reacted under identical experimental are disclosed in the companion cases Serial Nos. am 224,829 and 224,830 are applicable herein. Any TABLE; 11 basic catalyst and any of these organic activators can be used in a Wide variety of combinations. Percent It is to be further understood, however, that the if ff fi DMF 3 P Combined invention is applicable only to basic catalyzed dridc Acetate action ment fgf esterification. For acid catalyzed esterification of cellulose, the oven pretreatment has proven detrimental. 162 0 2.5 15 {g 3- H ven dried 25.14 co Although the preferred embodiment of the 152 0 2.5 at dried invention involves drying the cellulose in an 162 0 25 1 1cm d oven, other heat treating methods known to 33:00 those skilled in the art are applicable. As re- 81 71 15 Oven dried 38.40 gards the temperature of drying, studies indi- 81 11 2.5 30 8 32 dried 313 60 cate that temperatures ranging just above room 81 71 25 60 {None d temperature to about 150 C. and higher are m operative, depending upon the degree of activa- 1 tion desired. Generally speaking, the higher The results which are depicted in tne above table the temperature of the drying oven the more mdlcate the pronounced actlvatlon Whlch 55 effective the activation. Temperatures of about Qrlles from Oven f of cellulose (cotton 0. up to about 125 C. are operable afiording lmtels) acetylatmg the f m pres rapid moisture removal. Normally a temperaence of a bas c catalyst for periods of time ran tux-e of about C is used, temperatures of mg from 15 mmutes to one hour- 100 C. to C. being prefered. Temperatures 7o materially above C. are not used since at Example such elevated temperatures degradation and About 100 parts of pulp were dried at 105 for charring begin to occur. 60 hours. One-half of this dried material was Since the degree of activation is at least apsteeped in water and allowed to dry at room temproximately proportional to the temperature and perature to equilibrium moisture content. The 75 the time of drying, these two variables may be adjusted over a wide range to give satisfactory results. For example, a drying cycle of 16 hours is recommended when cotton linter cellulose is dried at 105 C.

The time of drying is, of course, dependent upon the amount of moisture initially present which varies from small amounts for unswollen cellulose to considerable amounts for swollen cellulose. Usually a time range of 2 to 4 up to to hours will suflice in the two instances, respectively, although shorter or longer times may be used.

Although the mechanism of the activation by heat process is not definitely settled, it is believed that the beneficial results achieved are due to a thermal change rather than a moisture removal. Heat treating cellulose may result in a certain amount of structural change (degradation) which is beneficial in terms of activation. It may involve the formation of condensed amorphous regions recently termed vitreous cellulose which are tightly bound through hydrogen bonding.

By practice of this invention, an efficient method of activating cellulose preparatory to basic catalyzed esterification is provided. The so-called heat pretreatment step allows esterification to proceed more rapidly and more readily to high combined acetic acid ranges. Furthermore, the oven-dried cellulose appears to react more smoothly with fatty acid anhydrides and less trouble with the formation of poorly reacted lumps is encountered. A greater tendency toward dispersion in the reaction medium is apparent. Other advantages include adaptability to continuous esterification procedures, as well as to processes involving the manufacture of various types of cellulose derivatives.

Any departure from the procedure described herein which conforms to the principles of the invention is intended to be included within the scope of the claims below.

I claim:

1. In the esterification of cellulose with acid anhydrides in the presence of a basic catalyst,

the improvement which comprises heating the cellulose prior to esterification at a temperature of about C. to about 125 C.

2. A process in accordance with claim 1 in which the said temperature is about C. to about C.

3. A process in accordance with claim 1 in which the time of heating is about 2 hours to about 20 hours.

4. A process for esterifying cellulose with acid anhydrides which comprises activating the cellulose prior to esterification by heating it at a temperature of about 75 C. to about 0.; and treating the resultant activated cellulose with an acid anhydride in the presence of a basic catalyst.

5. A process in accordance with claim 4 in which the said anhydride is acetic anhydride.

6. A process in accordance with claim 4 in which the said catalyst is potassium acetate.

7. A process for esterifying cellulose with acid anhydrides which comprises activating the cellulose prior to esterification by heating it at a temperature of about 75 C. to about 125 0.; and treating the resultant activated cellulose with an acid anhydride in the presence of a basic catalyst and in the presence of an organic activator selected from the group consisting of an amide and a lactone.

LEJAREN ARTHUR HILLER, JR,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,105,498 Parett et al. Jan. 18, 1938 2,289,085 Halewijn July '7, 1942 2,398,260 Kantorowicz Oct. 6, 1942 OTHER REFERENCES Ott, Cellulose and its Derivatives, 1943, page 676. 

1. IN THE ESTERIFLCATION OF CELLULOSE WITH ACID ANHYDRIDES IN THE PRESENCE OF A BASIC CATALYST, THE IMPROVEMENT WHICH COMPRISES HEATING THE CELLULOSE PRIOR TO ESTERIFICATION AT A TEMPERATURE OF ABOUT 75* C. TO ABOUT 125* C. 